It's a bit poor that Nero 5.5 writes DVD video, and the API which uses it appears to work, but it just doesn't.
Tried v6, and it does seem to work now. Only issue is since changing over, the DVDs I've made won't work in my PC DVD-R drive, but do work on the DVD-ROM drive (and my standalone DVD player).
I think part of the problem I had was setting the wrong flags. This what I have now...
m_NeroFiles.nwfscBurnOptions = NCITEF_CREATE_UDF_FS | NCITEF_CREATE_ISO_FS | NCITEF_USE_JOLIET | NCITEF_DVDVIDEO_REALLOC | NCITEF_DVDVIDEO_CMPT;
Is that correct? I've no idea what the realloc thing does, but watching the log in Nero that was one of the steps. And I didn't think the Joliet flag was an issue, but it seems to be on by default in Nero.